The present invention relates to an image scanning apparatus for obtaining an image from an object (e.g., a sheet of paper bearing an image) by separating a light signal consisting of a plurality of color components which are incident to and reflected from the object into respective color signals through a plurality of color filters corresponding to the respective color components.
In order to realize color reproduction suitable for the human visual system in an image processing apparatus, it is important to match the color signal in the range of visible light to human visual system. The typical response characteristic of the human visual system with respect to the wavelengths of red (R), green (G) and blue (B) has been outlined by the Commission International de I'Eclairage (CIE) as CIE-RGB chromacity diagram. Based on such chromacity diagram, a color image processor controls the spectral properties of three primary color signals R, G and B.
An image scanning apparatus (e.g., a scanner) obtains the image information of an object by separating the color image information thereof into color information consisting of three wavelength bands (red, green and blue) and quantifying the respective color information by means of an image sensor. This is commonly known as "color separation."
Generally speaking, color separation adopts two conventional methods which are largely classified into (1) a method which combines a black-and-white image sensor and a set of color filters whose bandpass characteristics correspond to the R, G and B wavelengths, and (2) a method using a color image sensor. The former method can minimize the cost for the embodiment of the apparatus but is difficult to realize technically. The latter method easily realize the constitution of the apparatus but bears an excessive cost and has certain limitations with regard to improvements in spectral characteristic.
Currently, the above former method is generally adopted and can be further subdivided into three separate methods. These are (1) a lamp switching method which uses one image sensor and separates colors by means of a plurality of light sources having different luminous spectral characteristics, (2) a filter switching method which uses one image sensor and one light source and separates colors by means of a color filter having bandpass characteristics corresponding to the R, G and B wavelengths, and (3) an optical path separation or prism mirror method which uses one light source, redirects the R, G and B optical paths by arranging an element (e.g., a prism) on an optical path having various refractive indices and separates colors by means of three image sensors.
Among the above three subdivided methods, the second (the filter switching method) is the best in terms of speed and cost. The filter switching method is again divided into a rotary filtering method which switches the filters by a rotating movement and a plate filtering method which switches the filters by a linear movement.
FIGS. 1A to 1D show embodiments of a color separating filter according to a conventional rotary filtering method, wherein arrows indicate the direction of movement of the rotary filters.
Rotary filter shown in FIG. 1A is disclosed in Japanese laid-open patent publication No. sho 61-294963. Here, a rotary filter is shown, wherein a set of red, green and blue color filters are installed on a rotary circular plate. In the case of a color image, the image information of the R, G and B wavelengths is obtained by rotating the rotary filter. For monochromatic images, the rotary filter is fixed and a single color filter is used to obtain image information.
Rotary filter shown in FIG. 1B is disclosed in Japanese laid-open patent publication No. sho 62-102690. Here, a rotary circular plate is installed in front of an image input apparatus. Then, a color filter for separating the three primary colors is attached to the rotary circular plate.
Rotary filter shown in FIG. 1C is disclosed in Japanese laid-open patent publication No. hei 2-89463 and is similar to that of FIG. 1A. Here, however, each color filter occupies a different sized area in order to maintain color balance.
Rotary filter shown in FIG. 1D is disclosed in U.S. Pat. No. 4,841,358 and shows a flat plate filter for switching filters by a rectilinear reciprocation.
However, none of these disclosures propose a specific device for calibrating a shading error due to the luminous characteristic of a light source and the vignetting characteristic of a lens. In general, a shading calibration is performed by an electrical circuit which can accomplish such calibration for shading errors of about 30% but no more. Moreover, even if the shading error is less than 30%, deterioration of the picture quality due to the calibration cannot be avoided.
FIG. 2 is a configurational diagram of the image scanning apparatus adopting a conventional rotary filter.
In the apparatus shown in FIG. 2, a light signal containing plural color components generated in a fluorescent lamp 20 is irradiated onto an image-bearing object 22 placed on a stand 21. The light signal reflected from image-bearing object 22 is incident to a line sensor 25 via a reflecting mirror 23 and a lens 24. The line sensor 25 generates an electrical signal proportional to the intensity of the incident light signal. A rotary filter 26 having a color filter 27 is installed between reflecting mirror 23 and lens 24. When rotary filter 26 is rotated by a driving motor 28, the respectively installed color filters 27a, 27b and 27c (FIG. 3) are sequentially interjected into the light path.
When an image is input by means of a line sensor, as in FIG. 2, one line of RGB information is input for every revolution of rotary filter 26. In the case of the image of an A4-sized sheet of paper (as prescribed by the International Organization for Standardization) being input with a resolution of 300 lines per inch, a rotation speed of 3,300 revolutions per minute is necessary to scan the sheet in one minute. To scan at higher speeds, rotary filter 26 should be rotated more rapidly. However, increasing the rotation speed results in a geometrical positioning error due to air friction of the filter and the unavoidable vibration of the rotating axis, and necessitates a cost increase for the pursuit of a high-quality driving motor.
FIG. 3 shows the relative position of a line sensor 25 with respect to the color filters 27a, 27b and 27c disposed on the rotary filter 26 of FIG. 2.
In the apparatus of FIG. 3, to input a subsequent line of image information after one line of image information is input, the sensor or the object to be scanned should be transported by one line interval. Given that color filters 27a, 27b and 27c are arranged equidistantly with respect to one another, since the transfer time of each filter cannot be established individually, the filter rotation is performed in conjunction with filter switching. As a result, a color registration error, whereby a pixel of a given point on the object cannot be matched with a pixel value of the corresponding location of R, G and B image information, is generated, thereby resulting in a geometrical distortion in the reproduced image. In particular, when an image consisting of a series of achromatic colors is input, the sharpness of the image contours is reduced, thereby resulting in a serious deterioration of picture quality.